U.S. patent application number 14/540608 was filed with the patent office on 2016-06-09 for crosstalk suppression in a multi-photodetector optical channel monitor.
The applicant listed for this patent is Nistica, Inc.. Invention is credited to Jefferson L. Wagener.
Application Number | 20160164600 14/540608 |
Document ID | / |
Family ID | 55955108 |
Filed Date | 2016-06-09 |
United States Patent
Application |
20160164600 |
Kind Code |
A1 |
Wagener; Jefferson L. |
June 9, 2016 |
CROSSTALK SUPPRESSION IN A MULTI-PHOTODETECTOR OPTICAL CHANNEL
MONITOR
Abstract
An optical device includes an optical port array having first
and second optical inputs for receiving optical beams and a first
plurality of optical outputs associated with switching
functionality and a second plurality of optical outputs associated
with channel monitoring functionality. A dispersion element
receives the optical beam from an input and spatially separates the
beam into a plurality of wavelength components. The focusing
element focuses the wavelength components. The optical path
conversion system receives the plurality of wavelength components
and selectively directs each one to a prescribed one of the optical
ports. The photodetectors are each associated with one of the
optical outputs in the second plurality of optical outputs and
receive a wavelength component therefrom. The controller causes the
optical path conversion system to simultaneously direct each of the
wavelength components to a different one of the optical outputs of
the second plurality of optical outputs.
Inventors: |
Wagener; Jefferson L.;
(Morristown, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nistica, Inc. |
Bridgewater |
NJ |
US |
|
|
Family ID: |
55955108 |
Appl. No.: |
14/540608 |
Filed: |
November 13, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14220583 |
Mar 20, 2014 |
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14540608 |
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61803524 |
Mar 20, 2013 |
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Current U.S.
Class: |
398/34 |
Current CPC
Class: |
G02B 6/35 20130101; H04J
14/0212 20130101; G02B 6/29391 20130101; H04B 10/07955 20130101;
G02B 6/3548 20130101; G02B 6/356 20130101; G02B 6/29373 20130101;
G02B 6/29316 20130101; G02B 6/293 20130101 |
International
Class: |
H04B 10/079 20060101
H04B010/079 |
Claims
1. A method for monitoring optical channels of a WDM optical
signal, comprising: a. simultaneously directing each channel in a
first sequence of N channels of the WDM optical signal to a
different one of N photodetectors, where N is greater than or equal
to 2 and the WDM optical signal includes M channels, where M>N;
b. simultaneously measuring a power level of each of the N channels
in the first sequence using the N photodetectors to obtain N
measured power levels; c. repeating steps (a) and (b) for one or
more additional sequences of N channels of the WDM optical signal
until the power level of each of the M channels has been measured
by one of the photodetectors.
2. The method of claim 1 further comprising performing additional
power level measurements using the photodetectors for correcting
the N measured power levels of the N channels to account for
crosstalk arising among the channels.
3. The method of claim 2 wherein the additional power level
measurements include measuring a power level received by each of
the N photodetectors when no channels are routed to any of the
photo detectors.
4. The method of claim 3 wherein the additional measurements
further include measuring a power level received by each of the
photodetectors when a different block of M/N channels are
sequentially routed to each one of the photodetectors to thereby
obtain (N+1) additional power level measurements.
5. The method of claim 1 further comprising developing a crosstalk
matrix having entries that each specify a contribution to crosstalk
by one photodetector to another of the photodetectors.
6. The method of claim 5 wherein the crosstalk matrix is a
(M/N+(N+1)).times.N matrix.
7. An optical device, comprising: an optical port array having at
least first and second optical inputs for receiving optical beams
and at least a first plurality of optical outputs associated with
switching functionality and a second plurality of optical outputs
associated with channel monitoring functionality; a dispersion
element receiving the optical beam from an optical input and
spatially separating the optical beam into a plurality of
wavelength components; a focusing element for focusing the
plurality of wavelength components; an optical path conversion
system for receiving the plurality of wavelength components and
selectively directing each of the wavelength components to a
prescribed one of the optical ports; a plurality of photodetectors
each associated with one of the optical outputs in the second
plurality of optical outputs for receiving a wavelength component
therefrom; and a controller for causing the optical path conversion
system to: a. simultaneously direct each wavelength component in a
first sequence of N channels of the WDM optical signal to a
different one of N photodetectors, where N is greater than or equal
to 2 and the WDM optical signal includes M wavelength components,
where M>N; b. simultaneously measure a power level of each of
the N wavelength components in the first sequence using the N
photodetectors to obtain N measured power levels; c. repeat steps
(a) and (b) for one or more additional sequences of N wavelength
components of the WDM optical signal until the power level of each
of the M wavelength components has been measured by one of the
photodetectors.
8. The optical device of claim 7 wherein the second plurality of
optical outputs includes N optical outputs, N being an integer
greater than or equal to 2, the controller being further configured
to cause the optical path conversion system to simultaneously
direct a first sequence of N wavelength components to one of the N
optical outputs and, subsequent thereto, simultaneously direct a
second sequence of N wavelength components to one of the N optical
outputs.
9. The optical device of claim 7 wherein the optical path
conversion system includes a programmable optical phase
modulator.
10. The optical device of claim 9 wherein the programmable optical
phase modulator has an axis normal to a plane in which it extends
that is nonparallel to a direction along which the optical beams
exit and enter the optical port array.
11. The optical device of claim 7 wherein the programmable optical
phase modulator is a liquid crystal on silicon (LCoS) device.
12. The optical device of claim 7 wherein the controller is further
configured to cause the optical path conversion system to perform
additional power level measurements using the photodetectors for
correcting the N measured power levels of the N channels to account
for crosstalk arising among the channels.
13. The optical device of claim 12 wherein the controller is
further configured to cause the optical path conversion system to
further measure a power level received by each of the N
photodetectors when no channels are routed to any of the
photodetectors.
14. The optical device of claim 13 wherein the additional
measurements further include a measurement of a power level
received by each of the photodetectors when a different block of
M/N channels are sequentially routed to each one of the
photodetectors to thereby obtain (N+1) additional power level
measurements.
15. The optical device of claim 7 wherein the controller is further
configured to develop a crosstalk matrix having entries that each
specify a contribution to crosstalk by one photodetector to another
of the photodetectors.
16. The optical device of claim 15 wherein the crosstalk matrix is
a (M/N+(N+1)).times.N matrix.
Description
BACKGROUND
[0001] Fiber optic communication systems typically employ
wavelength division multiplexing (WDM), which is a technique for
using an optical fiber to carry many spectrally separated
independent optical channels. In a wavelength domain, the optical
channels are centered on separate channel wavelengths which in
dense WDM (WDM) systems are typically spaced apart by 25, 50, 100
or 200 GHz. Information content carried by an optical channel is
spread over a finite wavelength band, which is typically narrower
than the spacing between channels.
[0002] Optical channel monitoring is increasingly being used by
telecommunications carriers and multi-service operators of fiber
optic systems. As the traffic on optical networks increases,
monitoring and management of the networks become increasingly
important issues. To monitor the network, the spectral
characteristics of the composite signal at particular points in the
network must be determined and analyzed. This information may then
be used to optimize the performance of the network. Optical channel
monitoring is particularly important for modern optical networks
that use reconfigurable and self-managed fiber-optic networks.
[0003] For example, reconfigurable optical add/drop multiplexers
(ROADMs) and optical cross connects, which are used to manipulate
individual wavelength channels as they are transmitted along the
network, require an optical channel monitor. A ROADM allows dynamic
and reconfigurable selection of wavelength channels that are to be
added or dropped at intermediate nodes along the network. In a
ROADM, for instance, an optical channel monitor can provide an
inventory of incoming channels as well as an inventory of outgoing
channels and to provide channel-power information to variable
optical attenuator (VOA) control electronics so that the power of
added channels can be equalized with the pass-through channels.
[0004] One type of optical channel monitor employs a wavelength
selective switch (WSS), which is a type of switch configured to
perform optical switching on a per wavelength channel basis, and is
typically capable of switching any wavelength channel at an input
fiber to any desired output fiber. Thus, a 1.times.N WSS can switch
any wavelength channel of the WDM input signal propagating along
the input fiber to any of the N output fibers coupled to the
WSS.
[0005] U.S. Pat. Appl. Publ. No. 2010/0046944 shows an optical
channel monitor that is incorporated in a WSS. This is accomplished
by using the functionality of a 1.times.1 switch that is available
in a 1.times.N WSS. In particular, the output of the 1.times.1
switch terminates with a photodiode. In this way, the power of any
individual channel can be measured.
[0006] While the use of a 1.times.1 WSS to form an OCM is useful
when the optical switching technology is sufficiently fast, this
technique is not suitable when used with switches that do not have
relatively fast response times. In particular, the optical
switching time, the photodiode settling time and the number of
channels being monitored determine the OCM loop speed, i.e., the
time needed to monitor each channel one time. For many applications
OCM loop speeds of less than 1 second, and ideally less than 0.1
second, are desired. Accordingly, the switch and photodiode
settling times need to be sufficiently fast to interrogate many
channels, which may approach or even exceed 100 in number. To
accomplish a 0.2 second loop speed with a photodiode settling time
of 1 ms and 100 channels, the optical switching time must also be 1
ms. While this is feasible with some technologies such as digital
micro-mirror devices (DMDs) it is not practical with other
technologies such as liquid crystal and Liquid Crystal on Silicon
(LCoS) technologies.
SUMMARY
[0007] In accordance with one aspect of the invention, an optical
device is provided. The optical device includes an optical port
array, a dispersion element, a focusing element, an optical path
conversion system, a plurality of photodetectors and a controller.
The optical port array has at least first and second optical inputs
for receiving optical beams and at least a first plurality of
optical outputs associated with switching functionality and a
second plurality of optical outputs associated with channel
monitoring functionality. The dispersion element receives the
optical beam from an optical input and spatially separates the
optical beam into a plurality of wavelength components. The
focusing element focuses the plurality of wavelength components.
The optical path conversion system receives the plurality of
wavelength components and selectively directs each of the
wavelength components to a prescribed one of the optical ports. The
plurality of photodetectors are each associated with one of the
optical outputs in the second plurality of optical outputs and
receive a wavelength component therefrom. The controller causes the
optical path conversion system to simultaneously direct each of a
plurality of wavelength components to a different one of the
optical outputs of the second plurality of optical outputs.
[0008] In accordance with another aspect of the invention, a method
is provided for monitoring the optical channels of a WDM optical
signal. The method includes (a) simultaneously directing each
channel in a first sequence of N channels of the WDM optical signal
to a different one of N photodetectors, where N is greater than or
equal to 2 and the WDM optical signal includes M channels, where
M>N; (b) simultaneously measuring a power level of each of the N
channels in the first sequence using the N photodetectors to obtain
N measured power levels and (c) repeating steps (a) and (b) for one
or more additional sequences of N channels of the WDM optical
signal until the power level of each of the M channels has been
measured by one of the photodetectors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 shows a functional block diagram of one example of a
wavelength selective switch (WSS) that includes an integrated
channel monitor.
[0010] FIG. 2 illustrates one example of a sequence that may be
used in connection with a device having a series of N (where N is
equal to or greater than 2) WSSs each having 5 output ports and an
OCM having N photodiodes receiving light from N output ports.
[0011] FIGS. 3A and 3B are top and side views respectively of one
example of a simplified optical device such as a free-space switch
that may be used in conjunction with embodiments of the present
invention.
[0012] FIG. 4 is a side view of an alternative example of a
simplified optical device such as a free-space switch that may be
used in conjunction with embodiments of the present invention.
[0013] FIG. 5 shows a sequence of 8 sets of simultaneous
measurements which are obtained to account for crosstalk when a WDM
signal having 12 channels are routed to an optical channel monitor
that employs three photodetectors.
DETAILED DESCRIPTION
[0014] FIG. 1 shows a functional block diagram of one example of a
wavelength selective switch (WSS) 100 that includes an integrated
channel monitor. As shown, three distinct functions are depicted:
two 1.times.n WSSs, represented by WSSs 110 and 120, and an optical
channel monitor 130 (OCM). It should be noted, however, that as
will be described below, the different functions may be
incorporated into a single physical switching device.
[0015] WSS 110 includes an input port 112 and output ports 1141,
1142, 1143, 1144 and 1145 ("114"). A switching fabric 116 optically
couples the input port 112 to the output ports 114 so that an
optical signal received at the input port 112 can be selectively
directed to one of the output ports 114 under the control of a
switch controller 140. Similarly, WSS 120 includes an input port
122 and output ports 124.sub.1, 124.sub.2, 124.sub.3, 124.sub.4 and
124.sub.5 ("124"). A switching fabric 126 optically couples the
input port 122 to the output ports 124 so that an optical signal
received at the input port 122 can be selectively directed to one
of the output ports 124 under the control of the switch controller
140.
[0016] OCM 130 is similar to WSSs 120 and 130 except that each of
its output ports terminates in a photodetector such as a
photodiode. In particular, OCM 130 includes an input port 132 and
output ports 134.sub.1, 134.sub.2, 134.sub.3, 134.sub.4 and
134.sub.5 ("134"). A switching fabric 136 optically couples the
input port 132 to the output ports 134 so that an optical signal
received at the input port 132 can be selectively directed to one
of the output ports 134 under the control of the switch controller
140. Photodiodes 150.sub.1, 150.sub.2, 150.sub.3, 150.sub.4 and
150.sub.5 receive light from optical outputs 134.sub.1, 134.sub.2,
134.sub.3, 134.sub.4 and 134.sub.5, respectively.
[0017] It should be noted that while the WSSs 110 and 120 and the
OCM 130 are depicted as having five output ports, more generally
any number of output ports may be employed, and this number may be
the same or different among the three functional elements. That is,
WSS 110, WSS 120 and OCM 130 may have the same or a different
number of output ports.
[0018] Because the OCM has multiple output ports that are each
equipped with a photodiode, multiple channels can be monitored
simultaneously, thereby increasing the OCM loop speed. For
instance, with only 1 photodiode, a 100-channel measurement would
take 100 sequential samples with switch and settle times between
each sample. If, for instance, a 1.times.20 WSS with 20 photodiodes
were used, then each photodiode could be sampled nearly
simultaneously, with 20 channels being detected in parallel. This
would reduce the loop time by a factor of 20 from in comparison to
time needed with a conventional arrangement. In this way a target
loop time of 0.2 seconds with a settling time of 1 ms could support
a switching time of 39 ms. Such a switching time is practical for
use with liquid crystal-based switching technologies.
[0019] Individual channels may be simultaneously routed to the OCM
130 for monitoring in a wide variety of different ways. FIG. 2
illustrates one example of a sequence that may be used in
connection with a device having a series of N (where N is equal to
or greater than 2) WSSs each having 5 output ports and an OCM
having N photodiodes receiving light from N output ports. As shown,
channels wavelengths 1, 2, 3, 4 and 5 are routed in sequence to the
five outputs of the first WSS. Wavelengths 6, 7, 8, 9 and 10 are
routed in sequence to the five outputs of the second WSS. This
process continues for each WSS, with the final wavelengths N, N+1,
N+2, N+3, N+4 and N+5 being routed in sequence to the five outputs
of the N.sup.th WSS.
[0020] Since the OCM has N outputs, one channel from each of the N
WSSs can be monitored simultaneously. For instance, with such an
arrangement channels or wavelengths 1, 6, 11, 16 . . . N can be
simultaneously monitored. Then, after these channels have been
monitored, channels 2, 7, 12, 17 . . . N+1 can be simultaneously
monitored, followed by channels 3, 8, 13, 18 . . . N+2, and so on.
Finally, the monitoring sequence may be completed by simultaneously
monitoring channels 5, 10, 15, 20 . . . N+4, after which the entire
sequence may be repeated. Accordingly, if there are a total of M
channels in a WDM optical signal to be monitored, M/N different
sets of N channels need to be measured to monitor each channel in
the WDM optical signal one time.
[0021] For many applications it may be cost prohibitive to build a
multi-port WSS that is solely dedicated for use as an OCM with
multiple photodiodes. However, the cost diminishes substantially if
the functionality of an OCM could be incorporated as an adjunct to
a device that includes the functionality of one or more WSS if most
of the optical elements used in the WSS(s) are also used to
implement the functionality of the OCM. In this case, the
incremental cost of the additional WSS can be small, making an OCM
having multiple photodiodes a viable alternative.
Illustrative Wavelength Selective Switch
[0022] One example of a wavelength selective switch in which an
optical channel monitor of the type described above may be
incorporated will be described with reference to FIGS. 3-4.
Additional details concerning this optical switch may be found in
co-pending U.S. application Ser. No. ______ [Docket No. 2062/17]
entitled "Wavelength Selective Switch Employing a LCoS Device and
Having Reduced Crosstalk."
[0023] FIGS. 3A and 3B are top and side views respectively of one
example of a simplified optical device such as a free-space WSS 100
that may be used in conjunction with embodiments of the present
invention. Light is input and output to the WSS 100 through optical
waveguides such as optical fibers which serve as input and output
ports. As best seen in FIG. 3B, a fiber collimator array 101 may
comprise a plurality of individual fibers 120.sub.1, 120.sub.2 and
120.sub.3 respectively coupled to collimators 102.sub.1, 102.sub.2
and 102.sub.3. Light from one or more of the fibers 120 is
converted to a free-space beam by the collimators 102. The light
exiting from port array 101 is parallel to the z-axis. While the
port array 101 only shows three optical fiber/collimator pairs in
FIG. 1B, more generally any suitable number of optical
fiber/collimator pairs may be employed.
[0024] A pair of telescopes or optical beam expanders magnifies the
free space light beams from the port array 101. A first telescope
or beam expander is formed from optical elements 106 and 107 and a
second telescope or beam expander is formed from optical elements
104 and 105.
[0025] In FIGS. 3A and 3B, optical elements which affect the light
in two axes are illustrated with solid lines as bi-convex optics in
both views. On the other hand, optical elements which only affect
the light in one axis are illustrated with solid lines as
plano-convex lenses in the axis that is affected. The optical
elements which only affect light in one axis are also illustrated
by dashed lines in the axis which they do not affect. For instance,
in FIGS. 3A and 3B the optical elements 102, 108, 109 and 110 are
depicted with solid lines in both figures. On the other hand,
optical elements 106 and 107 are depicted with solid lines in FIG.
3A (since they have focusing power along the y-axis) and with
dashed lines in FIG. 3B (since they leave the beams unaffected
along the x-axis). Optical elements 104 and 105 are depicted with
solid lines in FIG. 3B (since they have focusing power along the
x-axis) and with dashed lines in FIG. 3A (since they leave the
beams unaffected in the y-axis).
[0026] Each telescope may be created with different magnification
factors for the x and y directions. For instance, the magnification
of the telescope formed from optical elements 104 and 105, which
magnifies the light in the x-direction, may be less than the
magnification of the telescope formed from optical elements 106 and
107, which magnifies the light in the y-direction.
[0027] The pair of telescopes magnifies the light beams from the
port array 101 and optically couples them to a wavelength
dispersion element 108 (e.g., a diffraction grating or prism),
which separates the free space light beams into their constituent
wavelengths or channels. The wavelength dispersion element 108 acts
to disperse light in different directions on an x-y plane according
to its wavelength. The light from the dispersion element is
directed to beam focusing optics 109.
[0028] Beam focusing optics 109 couple the wavelength components
from the wavelength dispersion element 108 to a optical path
conversion system. In this example the optical path conversion
system is a programmable optical phase modulator, which may be, for
example, a liquid crystal-based phase modulator such as a LCoS
device 110. The wavelength components are dispersed along the
x-axis, which is referred to as the wavelength dispersion direction
or axis. Accordingly, each wavelength component of a given
wavelength is focused on an array of pixels extending in the
y-direction. By way of example, and not by way of limitation, three
such wavelength components having center wavelengths denoted
.lamda..sub.1, .lamda..sub.2 and .lamda..sub.3 are shown in FIG. 3A
being focused on the LCoS device 110 along the wavelength
dispersion axis (x-axis).
[0029] As best seen in FIG. 3B, after reflection from the LCoS
device 110, each wavelength component can be coupled back through
the beam focusing optics 109, wavelength dispersion element 108 and
optical elements 106 and 107 to a selected fiber in the port array
101. As discussed in more detail in the aforementioned co-pending
U.S. application, appropriate manipulation of the pixels in the
y-axis allows selective independent steering of each wavelength
component to a selected output fiber.
[0030] In one particular embodiment, the LCoS 110 is tilted about
the x-axis so that it is no longer in the x-y plane and thus is no
longer orthogonal to the z-axis along which the light propagates
from the port array 101. Stated differently, a skewed angle is
formed between the z-axis and a direction in the plane of the
modulator perpendicular to the wavelength dispersion axis. Such an
embodiment is shown in FIG. 4, which is a side-view similar to the
side-view shown in FIG. 3B. In FIG. 4 and FIGS. 3A and 3B, like
elements are denoted by like reference numerals. By tilting the
LOCS 110 in this manner crosstalk arises from scattered light can
be reduced.
[0031] While the optical path conversion system employed in the
particular wavelength selective switch shown in FIGS. 3-4 is based
on a programmable optical phase modulator (e.g., a LCoS device),
more generally other technologies may be employed instead,
including, for instance, MEMs-based devices such as DMDs.
Crosstalk
[0032] One problem that arises when an OCM having multiple
photodetectors is employed to simultaneously monitor multiple
channels is that optical crosstalk between the photodiodes can
cause inaccurate channel measurements. This may be a particularly
serious problem when a LCoS device is employed as the switching
technology.
[0033] To mitigate this undesired crosstalk, additional
measurements can be made as part of the sequence of channel
measurements described above. These additional measurements attempt
to directly measure the crosstalk between the PDs and use this
information to refine the estimate of the optical power on a
channel. As explained below, these additional measurements, which
are a direct measure of crosstalk between the photodiodes, scale as
the number of PDs, not the number of channels. Hence, even with
these additional measurements the technique still results in a
reduction in the OCM loop time that the use of multiple PDs
delivers.
[0034] The additional measurements that need to be taken to account
for reducing crosstalk are described below.
[0035] One measurement determines the power received by each of the
photodetectors when no channels are routed to any of the PDs. This
requires one additional set of simultaneous measurements to be
taken from the N PDs. Another measurement determines the power
received by each of the photodetectors when a block of channels is
sent to one and only one of the PDs without routing any channels to
any of the other PDs. This latter measurement is repeated by
sending, in turn, a block of channels to each individual PD without
sending any channels to any other PD. These latter simultaneous
measurements are thus performed N times. Thus, in total, an
additional N+1 simultaneous sets of measurements need to be taken
from the N PDs.
[0036] The series of measurements that need to be taken can be best
illustrated by an example. Suppose an OCM has 3 photodetectors and
an optical signal has 12 total channels to be measured. That is,
for this example N=3 and M=12. Accordingly, M/N=4 sets of
simultaneous measurements are needed to monitor the complete set of
channels and N+1=4 additional sets of simultaneous measurements are
needed to take crosstalk into account. Accordingly, in this example
a total of 8 sets of simultaneous measurements need to be
taken.
[0037] The sequence of the 8 sets of simultaneous measurements may
be as follows:
1: No channels routed to any PDs. 2: Channel 1 routed to PD 1,
Channel 5 routed to PD 2, Channel 9 routed to PD 3 3: Channel 2
routed to PD 1, Channel 6 routed to PD 2, Channel 10 routed to PD
4: Channel 3 routed to PD 1, Channel 7 routed to PD 2, Channel 11
routed to PD 3. 5: Channel 4 routed to PD 1, Channel 8 routed to PD
2, Channel 12 routed to PD 3. 6: Channels 1, 2, 3, 4 routed to PD
1, none routed to PD 2, none routed to PD 3 7: None routed to PD 1,
channels 5, 6, 7, 8 routed to PD 2, none routed to PD 3 8: None
routed to PD 1, none routed to PD 2, channels 9, 10, 11, 12 routed
to PD 3
[0038] This sequence of 8 sets of simultaneous measurements are
illustrated in FIG. 5, which shows the channels that are routed to
the three PDs in each of the steps. A dashed vertical line
indicates that the particular channel is not routed to the
particular photodetector during that step. A solid vertical line
indicates that the particular channel is routed to the particular
photodetector during that step. For example, in step 1, no channels
are routed to the three photodiodes when the first set of
measurements is made. Similarly, in step 2, channels 1, 5 and 9 are
respectively routed to Photodiodes 1, 2 and 3 when the second set
of measurements is made.
[0039] These 8 sequential measurements result in a total of 24
individual readings (3 PDs for 8 steps). The readings can then be
used as a set to determine an accurate channel power even when
there is significant optical crosstalk.
[0040] In one illustrative embodiment, the readings can be used to
develop a crosstalk matrix M having entries Ma,p that denote the
crosstalk contribution from all the PDs during step number a when
PD number p is being measured. This matrix is thus a
(M/N+(N+1)).times.N dimensional matrix. For instance, if the power
level of PD 1 is being measured, the contribution to this
measurement from power routed to PD 2 is proportionally equal to
X1,2=M7,1/M7,2. Similarly, the crosstalk contribution to the power
level of PD1 from power routed to PD3 is proportionally equal to
X1,3=M8,1/M8,3. The crosstalk contribution to the measurement when
no channels are routed is proportionally equal to
X1,0=M1,1/(M6,1+M7,2+M8,3). In general these ratios should not be
allowed to exceed 1.
[0041] Using these ratios for PD1, and equivalent ones for the
other PDs, a corrected channel power level can be iteratively found
solving the following series of simultaneous equations:
M k = A K + i = 0 , i .noteq. k n X I A i ##EQU00001##
[0042] Where M.sub.k is the measured or detected power of channel k
and A.sub.k is actual power level of channel k after the impact of
crosstalk is reduced or eliminated.
* * * * *